Diabetes Volume 65, March 2016 539

Pierre-Louis Tharaux1,2,3,4 and Tobias B. Huber 4,5,6

How Is Proteinuric Diabetic Nephropathy Caused by Disturbed Proteostasis and Autophagy in Podocytes?

Diabetes 2016;65:539–541 | DOI: 10.2337/dbi15-0026

Progression of diabetic nephropathy (DN) is commonly depends on several genes including Map1lc3B/LC3B, defined by an increase in albuminuria from normoalbu- Becn1/Beclin-1, and other autophagy-related (Atg) genes minuria to microalbuminuria and from microalbuminuria (4). Dysregulation of autophagy is involved in the patho- to macroalbuminuria. Although many therapeutic interven- genesis of a variety of metabolic and age-related diseases tions, including reducing hyperglycemia and intraglomerular (11–17). One caveat of many clinical studies dealing with pressure, have been shown to slow down the progression tissues that should be taken into consideration is that of DN, many patients still develop end-stage renal disease. there is a general tendency to extrapolate information A major difficulty in inducing remission in patients with regarding the levels of autophagy substrates to the levels early DN is the identification of biomarkers that could of autophagy flux within the tissues. Despite the scarce COMMENTARY help to identify patients more likely to progress to end- but compelling literature on the roles of autophagy in the stage renal disease. Traditional risk factors, such as albumin- resistance to DN, studies need to determine whether uria, do not effectively predict DN progression, and other autophagy genes and markers are suitable biomarkers or predictors of DN have yet to be characterized and validated. targets for therapeutic intervention to ameliorate the progres- The need for discovering sensitive and robust biomarkers sion of DN. Future research would ultimately determine the to monitor the decline in renal function and to separate most reliable method for alterations of autophagy related to progressors from nonprogressors of DN is therefore of DN progression, taking ease and quantity of sample acquisi- paramount importance. tion into account. Next to mesangial extracellular matrix deposition and The function of autophagy in the kidneys is currently a thickening of basement membranes, progressive loss of under investigation, and it has been shown to have a glomerular pericytes and “podocytes” and microvascular renoprotective effect in several animal models of aging alterations appear to most closely correlate with the func- and acute kidney injury, especially in glomeruli (18–21). tional renal decline in DN (1–3). Importantly, postmitotic podocytes exhibit high levels of Autophagy (“self-eating” in Greek) is a highly regulated basal autophagy as a key regulator of podocyte and glo- lysosomal protein degradation pathway that removes pro- merular maintenance (22) (Fig. 1). tein aggregates and damaged or excess organelles in order to In this issue of Diabetes, Tagawa et al. (23) confirm the maintain intracellular homeostasis and cell integrity (4–6). protecting role of podocyte autophagy on DN, as selective This process was first described in 1957 by Sam Clark Jr. (7), targeting of the Atg5 alleles in these cells accentuated but the term “autophagy” was coined in 1963 by Christian experimental high-fat diet–induced DN as previously ob- de Duve (8). Autophagy process is well conserved in the served in a model of type 1 DN (24). Furthermore, it is evolution from yeast to mammals in various cell types in interesting that the loss of autophagy in podocytes affects many organs (9,10). The formation of autophagosomes the ultrastructure and function of these cells but also that

1Paris Cardiovascular Centre, INSERM, Paris, France Corresponding author: Pierre-Louis Tharaux, [email protected], or 2Université Paris Descartes, Sorbonne Paris Cité, Paris, France Tobias B. Huber, [email protected]. 3 – Nephrology Service, Georges Pompidou European Hospital, Assistance Publique © 2016 by the American Diabetes Association. Readers may use this article as Hôpitaux de Paris, Paris, France long as the work is properly cited, the use is educational and not for profit, and 4 FRIAS, Freiburg Institute for Advanced Studies and Center for Biological System the work is not altered. Analysis-ZBSA, Freiburg, Germany See accompanying article, p. 755. 5Renal Division, University Hospital Freiburg, Freiburg, Germany 6BIOSS Center for Biological Signalling Studies, Albert-Ludwigs-University Freiburg, Freiburg, Germany 540 Commentary Diabetes Volume 65, March 2016

shown to be of critical importance for the limitation of progression of DN (24). There is a strong relationship between endothelial dysfunction and DN in humans (25,26), and an increase in urinary albumin excretion in combination with rising blood pressure is a major risk factor for cardiovascular morbidity (27–30). The mecha- nism for such cross talk between the glomerular filtration barrier and other vascular beds is unclear and represents a major pathophysiological question. Fundamentally, the work by Tagawa et al. (23) also adds a major question mark about the pathophysiological importance of protein homeostasis, also called “proteostasis,” in terminally differentiated cells such as podocytes. Different mechanisms are involved in proteostasis, among them degradation systems (the main intracellular proteolytic systems being proteasome and lysosomes), folding systems (including molecular chaperones), and enzymatic mechanisms of protein repair (31). Tagawa et al. report the accumulation Figure 1—Postmitotic podocytes exhibit high levels of basal auto- phagy as a key regulator of podocyte and glomerular maintenance. of large lysosomes with an increase in lamp2-positive areas Elevation of glucose concentration inhibits podocyte autophagy (—). and an absence of autophagosomes in podocytes from mas- Selective targeting of the Atg5 alleles in these cells disrupted auto- sively proteinuric and diabetic rats and in mice with ge- – phagosome formation and accentuated experimental high-fat diet netic targeting of Atg5, a gene required for autophagy. induced DN. Altered autophagy led to accumulation of damaged fi organelles and proteins and disturbance of lysosomes trafficking. Thus, metabolically and genetically driven autophagy de - Furthermore, insufficient podocyte autophagy was found in diabetic ciency was associated with dysfunctional lysosomes accu- patients and rats with massive proteinuria accompanied by podocyte mulated in podocytes. The preservation of lysosome- and loss, but not in those with no or minimal proteinuria. The nature of the autophagy-mediated proteostasis may be critical for podo- negative signal associated with albuminuria is not known. Endocy- tosed albumin (green dots) per se may saturate podocyte autophagy cytes to cope with increased amounts of damaged proteins or albuminuria may be associated with local passage or secretion of produced during diabetes, a condition favoring increased inhibitory mediators (purple dots) that remain to be identified. Such intensity of nonenzymatic modifications of amino acids circulating mediator may also impact endothelial autophagy that fi has also been shown to limit progression of experimental DN. Red (32) and high incidence of posttranslational modi cations dots represent phosphatidyl-ethanolamine–conjugated microtubule- of proteins as reported in b-cells and nerves (33,34). associated protein 1A/1B-light chain 3 (LC3), which is recruited to In conclusion, the study by Tagawa et al. (23) adds to autophagosomal membranes and helps to monitor autophagy flux. the recent corpus of evidence that therapeutic stimulation EC, endothelial cells; GBM, glomerular basement membrane. or at least maintenance of autophagy and proteostasis represents an important nascent field of research to pre- of nearby mesangial cells, which become sclerotic. These vent and treat complications of diabetes. data underline the communication between podocytes and mesangial cells, which deserve further studies in the field of glomerular diseases. Funding. This work was supported by INSERM (to P.-L.T.), the Joint “ Importantly, Tagawa et al. (23) also show insufficient Transnational Call 2011 for Integrated Research on Genomics and Pathophys- iology of the Metabolic Syndrome and the Diseases arising from it” from podocyte autophagy in patients with diabetes and rats L’Agence Nationale de la Recherche of France (to P.-L.T.), and the German with massive proteinuria accompanied by podocyte loss Research Foundation (to T.B.H.). This study was further supported by the German but not in those with no or minimal proteinuria. The Research Foundation’s SFB 1140 Kidney Disease—From Genes to Mechanisms, causes of such alteration of podocyte autophagy are poten- Heisenberg program, and grant CRC 992 (to T.B.H.); by the European Research tially several. Meanwhile, stimulation of cultured podocytes Council (to T.B.H.); and by the Excellence Initiative of the German Federal and with sera from patients with diabetes or rats with massive State Governments (EXC 294 to T.B.H.). proteinuria-impaired autophagy resulted in apoptosis. Duality of Interest. No potential conflicts of interest relevant to this article These findings might be of crucial importance as they were reported. suggest the existence of serum factors promoting podo- References cyte stress and dysfunction with blunting of autophagy in proteinuric individuals. Let us hope this provocative find- 1. Ziyadeh FN, Hoffman BB, Han DC, et al. Long-term prevention of renal insufficiency, excess matrix gene expression, and glomerular mesangial matrix ing will lead to future work aimed at identifying such expansion by treatment with monoclonal antitransforming growth factor-beta factors. Another question rising from the study by Tagawa antibody in db/db diabetic mice. 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